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Titel |
Optimal pumping scenarios for the estimation of the saltwater intrusion front in the coastal aquifer of Tympaki, Crete - Greece |
VerfasserIn |
M. D. Dettoraki, Z. Dokou, E. A. Varouchakis, G. P. Karatzas |
Konferenz |
EGU General Assembly 2012
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 14 (2012) |
Datensatznummer |
250061160
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Zusammenfassung |
This study examines the saltwater intrusion problem, a naturally occurring phenomenon
that is often augmented by human activity through the over-exploitation of the
groundwater reserves. Saltwater intrusion occurs in most coastal aquifers due to the
constant contact with the saltwater and the imbalance of the freshwater / saltwater
that occurs under intense pumping conditions. The present study focuses on the
Municipality of Tympaki aquifer in Heraklion, Crete, where the phenomenon is particularly
intense. The main source of income in the region is agriculture, an activity that
demands large amounts of water, especially during the summer. In the Tympaki basin
there are 27 pumping wells with the majority of them located at the centre of the
basin.
The purpose of this study is to investigate the current extent of saltwater intrusion as well
as to find the optimal pumping scheme, for which the saltwater intrusion will retreat, while
meeting the irrigation needs of the region. A mathematical model that simulates the
groundwater flow is used, in order to study the size, extent and rate of the phenomenon.
MODFLOW was chosen for this purpose, which is a three-dimensional finite-difference
groundwater flow model. Initial conditions, boundary conditions, hydraulic conductivity
values and precipitation data for the basin were used in order to perform the model calibration
and simulation.
The optimization of the pumping sheme was performed using the GWM (Ground-Water
Management) model, which utilizes the simplex method. The optimization problem at hand is
slightly nonlinear because of the nonlinear relation of the free surface groundwater system to
the pumping stress. GWM solves the nonlinear problem using a sequential linearization
approach. At each step, the aquifer is considered confined and the response matrix of the
system is calculated for an initial set of pumping rates. The Simplex method is then used in
order to solve this linear problem and compute the optimal pumping rates for the 27 pumping
wells. The calculated pumping rates are used as initials for the next step of the algorithm
and new response coefficients are calculated. The process is terminated when the
difference between two successive solutions is less than a user-specified convergence
value.
The Ghyben-Herzberg equation was used to identify the position of the saltwater
intrusion front for the aquifer. Considering the aquifer depth at sea level to be 100 m the
seawater intrusion front was estimated to be extended in coastal regions with hydraulic heads
between 100 and 102.5 m. Initially, the optimal pumping rates that meet the water
demand in the region while preventing further intrusion were calculated. Three
additional scenarios were then considered in order to investigate the possibility of
retraction of the saltwater intrusion front towards the shoreline. These scenarios
seek to maximize pumping activity while allowing the saltwater intrusion front to
retreat towards the shoreline in pre-specified locations that fall between the worst
case scenario (current conditions) and best case scenario (zero pumping activity). |
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